Printing apparatus

Abstract: In a thermal printing apparatus, three consecutive image data items in the main scanning direction is sequentially extracted. When all the three image data items are at level 0 (indicating that no dot is printed) and at level 1 (indicating that a dot is printed), a printing density is determined to be 0 (indicating that no dot is printed) and 3 (indicating that a dot of the highest printing density is printed), respectively. When one of the three image data items is at level 1, the printing density is determined to be 1 (indicating that a dot of the lowest printing density is printed) when the printing density of the preceding printed pixel is any value other than 0. When one or two of the three image data items is or are at level 1, the printing density is determined to be 1 or 2 (indicating that a dot of an intermediate printing density is printed) according to the printing density of the preceding printed pixel. Thus, the printing apparatus is capable of printing image data without the degradation of the image quality, even when the image data whose resolution in the main scanning direction is higher than (e.g., three times as high as) that inherent to the apparatus is supplied.

Description: The present invention relates to a printing apparatus which, in response to supplied image data, prints a large number of dots arranged two-dimensionally on a printing sheet. Apparatuses of this type have their own maximum value of the printable pixel density (i.e., the pixel density). For example, in the case of the heat-transfer printing apparatus, the density of heating elements constituting a thermal head is the maximum value of the printable pixel density. Therefore, when image data whose pixel density is higher than the printable pixel density is given, this data, before printing, is converted into image data having as low a pixel density as can be printed. With this method, however, the decreased pixel density degrades the quality of the printed image significantly as compared with the original image data. When such an apparatus as a copier itself reads image data, a difference in the pixel density cannot take place. However, when such an apparatus as a facsimile receives image data from another unit, the aforesaid difference in the pixel density can occur. To prevent such a difference from occurring, an apparatus must be constructed so that printing may be effected at a pixel density equal to the highest density of the image data that can be supplied. For example, in the case of the heat-transfer printing apparatus, as the density of the heating elements of the thermal head is made higher, the pixels can be printed at a higher density accordingly. When the density of heating elements is made higher, the number of heating elements increases and the manufacture becomes more difficult, resulting in a costly thermal head. In addition, because a larger number of heating elements must be driven, the size of the driving circuit increases, thus making the circuit more expensive. Document EP 0 476 863 A1 discloses a printing apparatus as set out in the preamble of claim 1. In this apparatus, a emission-time setting device determines a period of time during which a drive section is to drive a laser diode for printing a pixel, based on k image data items stored in a shift register. As described above, a conventional thermal printing apparatus has a disadvantage in that when image data whose pixel density is higher than the printable pixel density is supplied, the quality of the printed image is seriously degraded as compared with that of the image indicated by the original image data. It has another disadvantage in that when the apparatus is constructed so that printing may be effected at a pixel density equal to the maximum pixel density of the image data that can be supplied, in order to always print at an image quality equal to that of the image indicted by the original image data, the manufacturing cost will rise. Accordingly, it is an object of the present invention to provide a printing apparatus capable of printing even an image whose pixel density is higher than the printable pixel density with less degradation of the image quality, while keeping the recordable pixel density as low as possible to prevent the cost from rising. According to the present invention, there is provided a printing apparatus which prints image data having a specific resolution by means of a printing head whose resolution in a main scanning direction is 1/k (k is a given integer) of that of the image data, the apparatus comprising means for extracting k consecutive image data items in the main scanning direction from the image data; means for determining a printing density of a pixel, and means for driving the printing head according to the printing density determined by the determining means. The means for determining a printing density are formed as set out in the characterising portion of Claim 1. With the printing apparatus of the present invention, even if image data whose pixel density is higher than the printable pixel density is supplied, it is possible to record an image whose quality is almost equal to that of the image indicated by the image data, without introducing a rise in the cost of the apparatus and an increase in the circuit size. Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the present invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the present invention in which: FIG. 1 is a schematic diagram of a heat-transfer printing apparatus according to a first embodiment of the present invention; FIG. 2 is a plan view showing a detailed configuration of a thermal head of the heat-transfer printing apparatus of the first embodiment; FIG. 3 is a block diagram of the electric circuitry of the heat-transfer printing apparatus of the first embodiment; FIG. 4 graphically shows rules which must be referred in determining the printing gradation density in the heat-transfer printing apparatus according to the first embodiment; FIGS. 5A and 5B show the original image and the image printed by the heat-transfer printing apparatus of the first embodiment for the sake of comparison; FIGS. 6A, 6B, and 6C show the original image, the reading gradation density (analog signal) at the position of scanning line "A" of the original image, and the printing density distribution on the printed image corresponding to the position of the scanning line "A"; A preferred embodiment of a printing apparatus according to the present invention will now be described with reference to the accompanying drawings. FIG. 1 is a sectional view showing the overall configuration of a heat-transfer printing apparatus according to a first embodiment of the present invention, for use in a facsimile or the like. Explaining here will be a case where the image data whose resolution is higher than the inherent resolution in the main scanning direction and equal to the inherent resolution in the sub-scanning direction is supplied to the heat-transfer printing apparatus. There is provided a linear thermal head 1 composed of a plurality of heating resistive elements arranged one-dimensionally in the main scanning direction. The recording surface of the head 1 is pressed against a platen roller 2 by means of a spring 3. In FIG. 1, the main scanning direction is a direction perpendicular to the drawing paper. An ink film 4 and a printing sheet 5, the film 4 on top of the sheet 5, pass between the thermal head 1 and the platen roller 2. The lateral direction of the printing sheet 5 is the main scanning direction. The thermal head 1 is supplied with a signal from a printing processor 20. The ink film 4 is wound around a supply roller 6. The leading edge of the ink film 4 is secured to a take-up roller 7. As the take-up roller 7 rotates, the ink film 4 fed from the supply roller 6 is used for printing and then wound around the take-up roller 7. In the vicinity of the platen roller 2, there are provided guide shafts 8a and 8b for keeping the passage of the ink film 4 in a suitable state. FIG. 2 is a plan view showing a detailed configuration of the thermal head 1 of FIG. 2. The thermal head 1 is constructed by arranging a large number of rectangular heating resistive elements 11 1 , 11 2 , ... one-dimensionally at regular intervals on an insulating substrate 10 with one end of each of the heating resistive elements 11 1 , 11 2 , ... connected to a common electrode 12, and the other end of each resistive element connected to individual electrodes 13 1 , 13 2 , respectively. FIG. 3 is a block diagram of the electric circuitry of the printing processor 20. The image data of each line (a line in the main scanning direction) externally supplied from, e.g., the receiving section of a facsimile machine or the reading section of a copier, is temporarily stored in a line buffer 21. The line buffer 21 outputs the stored image data at a specific rate. The dot data on each bit of the image data output from the line buffer 21 is supplied to latch memories 22 1 , 22 2 , ..., 22 n in sequence. Here, the number of latch memories n is determined as follows. Although the resolution (pixel density) in the main scanning direction of the thermal head 1 is determined by the pitch of heating resistive elements 11 1 , 11 2 , ..., the value of n is set so that even if the image data whose resolution in the main scanning direction is up to n times the determined resolution is supplied, it may be dealt with. Therefore, all latch memories 22 1 , 22 2 , ..., 22 n are not always used. When the resolution in the main scanning direction of the actually supplied image data is i times the resolution in the main scanning direction inherent to the head (i < n), each dot data on each bit is supplied only to i latch memories 22 1 , 22 2 , .., 22 i in sequence. This control is performed by a read/write controller 23. For example, the horizontal resolution of the supplied image data is three times as high as that of the thermal head, each dot data item on each bit is written in latch memories 22 1 , 22 2 , and 22 3 periodically. In other words, since the resolution of the thermal head 1 is one-third that of the image data, three dot data items of the image data must be expressed by using a single heating resistive element. To achieve this, the individual latch memories 22 1 , 22 2 , and 22 3 latch every third dot data item, so that the latch memories 22 1 , 22 2 , and 22 3 extract the dot data items on consecutive three pixels. The outputs of the latch memories 22 1 , 22 2 , ..., 22 n are supplied to a printing density determination circuit 25. The printing density determination circuit 25 determines the printing density of a pixel to be printed this time on the basis of the i original pixel data items extracted by latch memories 22 1 , 22 2 , ..., 22 i and the printing density of the preceding printed pixel, and outputs the printing density data. Namely, the printing density determination circuit 25 converts i original pixel data items (binary data items) into a single printing density data item (multi-value data item) of a printing pixel. The printing density data is supplied to a head driver 27 and is also held in a printing density data latch 26. The output of the latch 26 is fed back as the preceding printing density data to the printing density determination circuit 25. The head driver 27 drives each of the heating resistive elements 11 1 , 11 2 , ... of the thermal head 1 on the basis of the printing density data output from the printing density determination circuit 25. Now, the operation of the heat-transfer printing apparatus thus constructed will be explained. For convenience of explanation, it is assumed that the resolution in the main scanning direction of the image data externally supplied is three times as high as the resolution in the main scanning direction inherent to the head 1. First, one line of image data is temporarily stored in the line buffer 21 and the individual dot data items are written in latch memories 22 1 , 22 2 , and 22 3 in sequence. The printing density determination circuit 25 determines which of four levels, level 0 to level 3, the printing density of a pixel to be printed next should take as shown in FIG. 4, on the basis of the three original data items output from the latch memories 22 1 , 22 2 , and 22 3 and the printing density of the preceding printed pixel latched in the printing density data latch 26. The relationship between these four levels is: level 0 < level 1 < level 2 < level 3. FIG. 4 graphically shows a way to determine the printing density. In FIG. 4, the white portions of the original image data items indicate that a dot is absent (e.g., bit data = 0), whereas the black portions indicate that a dot is present (e.g., bit data = 1). It can be seen from FIG. 4 that the printing density is determined as follows: (1) When all of the three original data items indicate that a dot is absent (black dot ratio = 0/3), the printing density of a pixel to be printed next is determined to be at level 0. (2) When only one of the three original image data items indicates that a dot is present (black dot ratio = 1/3) and the preceding printing density L is at any level other than level 0, the printing density of a pixel to be printed next is determined to be at level 1. (3) When only one of the three original image data items indicates that a dot is present (black dot ratio = 1/3) and the preceding printing density L is at level 0, the printing density of a pixel to be printed next is determined to be at level 2. (4) When only two at both ends of the three original image data items indicate that a dot is present (black dot ratio = 2/3) and the preceding printing density L is at level 3, the printing density of a pixel to be printed next is determined to be at level 1. (5) When only two at both ends of the three original image data items indicate that a dot is present (black dot ratio = 2/3) and the preceding printing density L is at any level other than level 3, the printing density of a pixel to be printed next is determined to be at level 2. (6) When only two consecutive ones of the three original image data items indicate that a dot is present (black dot ratio = 2/3), the printing density of a pixel to be printed next is determined to be at level 2. (7) All of the three original image data items indicate that a dot is present (black dot ratio = 3/3), the printing density of a pixel to be printed next is determined to be at level 3. The printing density determination circuit 25 performs the above-mentioned determination process on the three original image data items extracted from the latch memories 22 1 , 22 2 , and 22 3 repeatedly, and sequentially supplies the printing density data on the pixels to be printed not only to the head driver 27 but also to the printing density data latch 26. The head driver 27 drives the thermal head 1 by controlling the amount of conducting energy to the individual heating resistive elements 11 1 , 11 2 , ... of the thermal head 1 according to each printing density data item. Here, because level 0 is a white level (at which no dot is printed), the amount of energy is 0. Level 1 is set to the amount of energy which itself is insufficient for printing a dot but, when added with a small amount of energy, can print a dot whose gradation density corresponds to a black dot ratio of 1/3. Level 2 is set to the amount of energy required for printing a dot whose density corresponds to a black dot ratio of 2/3. Level 3 is set to the amount of energy necessary for printing a normal black dot. The head driver 27 conducts electricity so that these amounts of energy may be supplied to the heating resistive elements 11 1 , 11 2 , .... The amount of energy supplied to the heating resistive elements 11 1 , 11 2 , ... can be changed according to the applied voltages to them and the conducting time. Converting three image data items (binary data items) into a single printing density data item (multi-level data item) in this way has the following meaning. Since the binary image data of three bits are converted so that the black dot ratio may not change, the printing density after the conversion has four levels. With the present invention, the black dot ratio of the original image data for three pixels is not always directly converted into the printing density level. As describe above, exceptions like items (3) and (4) are set up. This is done to reflect in the printing density the tendency of black dots to concentrate in the image data. Specifically, when only one of three original image data items indicates that a dot is present, a dot of level 1 with the lowest density is printed in the position corresponding to the three original image data items. However, when the preceding adjacent printed dot is at one of level 1 to level 3, this means that energy is being applied to the heating element corresponding to the adjacent dot. Under the influence of such energy, a dot of level 1 is printed. When the adjacent dot is at level 0, however, no dot is printed. Therefore, with the present invention, in the case of item (3) where only one of the three original image data items indicates that a dot is present, and the printing density of the preceding printed dot is at level 0, energy of level 2 is supplied to the head to print a dot of level 2. When only two of the three original pixel data items indicate that a dot is present, a dot of level 2 whose printing density corresponds to a black dot ratio of 2/3 is printed in the position corresponding to the three original image data items. However, when the original image data items indicating that a dot is present are separate at both ends of the three original image data items, the central original pixel indicating that a dot is absent is more likely to be a white thin line in the black ground. To prevent this white thin line from being lost, a dot of level 1, whose printing density is low, is printed. When all of the three original image data items indicate that a dot is present, a dot of level 3, whose printing density is the highest, is printed in the position corresponding to the three original image data items. With this method, for example, when the image data obtained by reading the image as shown in FIG. 5A is received and recording is effected, the image as shown in FIG. 5B is printed. As shown, dots of the highest printing density are printed in the central portion of the black image portion, assuring a sufficient black printing density. In the contour portion, the printing density of dots is processed so as to change stepwise in such a manner that the notched contour is inconspicuous. The reason why the notched contour is made inconspicuous by changing the printing density of dots stepwise in the contour portion as described above is as follows. When the image shown in FIG. 6A is read in the position of scanning line "A", an analog image signal representing the density is as shown in FIG. 6B. As seen from FIG. 6B, the read density in the contour portion changes gradually. On the other hand, since the printing density of the printed image produced by the heat-transfer printing apparatus of the present embodiment changes stepwise in the contour portion, the printing density distribution in the main scanning direction is as shown in FIG. 6C. As a result, the printing density distribution of the printed image approximates the density distribution of the image data so well that the contour portion looks smooth. As described above, with the first embodiment, the density distribution of an image is estimated on the basis of the image data, and the printing density of dots to be printed is changed so that the printing density distribution approximating the estimated distribution may be obtained, Therefore, although the pixel density (resolution) in the main scanning direction that can be achieved by the thermal head 1 is lower than that of the image data, the thermal head 1 can print a high-quality image approximating the image indicated by the original image data. This reduces the number of heat resistive elements constituting the thermal head 1, making its cost lower. Further, because there is no increase in the circuit size of the head driver 27, it is possible to keep the manufacturing cost low. While in the above explanation, the resolution in the main scanning direction of the image data supplied externally is three times the resolution in the main scanning direction inherent to the head 1, the present invention may, of course, be applied to the image data whose resolution in the main scanning direction is up to n times that inherent to the head 1. The thermal printing apparatus in the first embodiment is suitable for facsimile machines. This is because the facsimile machine can use image data with more than one resolution and there is a possibility that it will receive image data whose resolution is higher than the printable resolution. With a conventional facsimile machine, when the image data whose resolution is higher than the printable resolution arrives, the arrived image is sampled to convert it into image data with a printable resolution. Therefore, part of the transferred data must be discarded, wasting much of the data. Use of a thermal printing apparatus of the present invention can reduce such waste because it can use all the data arrived. As in the first embodiment, by determining the printing density of each dot, taking into account the printing density of adjacent dots (specifically, the preceding one) in the main scanning direction, a high-quality image can be printed if the image is made up of characters or line drawings having the nature of black or white dots to concentrate to some extent. As has been explained, according to the present invention, it is possible to provide a thermal printing apparatus capable of minimizing the degradation of the quality of an image whose resolution is higher than the printable resolution and achieving excellent printing, while keeping the printable resolution low to prevent a rise in the cost. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. For example, in the above embodiment, a thermal printing apparatus of the invention is applied to a heat-transfer printing apparatus, the present invention may be applied in the same manner to a heat-sensitive recording apparatus which uses heat-sensitive recording paper instead of an ink film. Further, the invention may be applied to an electrophotographic recording apparatus such as a laser printer.

Claims: A printing apparatus which prints image data having a specific resolution by means of a printing head whose resolution in a main scanning direction is 1/k (k is a given integer) of that of the image data, wherein k is a given integer, said apparatus comprising: means (22 1 , 22 2 , 22 3 , ....) for extracting k consecutive image data items in the main scanning direction from the image data; means (25) for determining a printing density of a pixel; and means (27) for driving the printing head according to the printing density determined by said determining means; characterized in that said means (25) are formed for determining a printing density of a pixel on the basis of a combination of the k image data items extracted by said extracting means and a printing density of the pixel immediately preceding the current pixel, such that the preceding printing density is considered for estimating the density distribution along the main scanning direction and the printing density of the current pixel is corrected in order to approximate the estimated distribution. An apparatus according to claim 1, characterized in that said image data is formed of binary image data items; and said determining means comprises means for converting k image data items into a printing density of (k+1) level. An apparatus according to claim 1, characterized in that the resolution of said printing head is one-third that of the image data; and said determining means determines the printing density to be 0 indicating that no dot is printed when three image data items are all at level 0 indicating that no dot is printed, the printing density to be 1 indicating that a dot of a lowest printing density is printed when the printing density of the preceding printed pixel is any value other than 0 and one of three image data items is at level 1 indicating that a dot is printed, the printing density to be 2 indicating that a dot of an intermediate printing density is printed when the printing density of the preceding printed pixel is 0 and one of three image data items is at level 1, the printing density to be 1 when the printing density of the preceding printed pixel is 3 and discrete two of three image data items are at level 1, the printing density to be 2 when the printing density of the preceding printed pixel is any value other than 3 and discrete two of three image data items are at level 1, the printing density to be 2 when two consecutive ones of three image data items are at level 1, and the printing density to be 3 indicating that a dot of a highest printing density is printed when three image data items are all at level 1. An apparatus according to claim 1, characterized in that said printing head is a thermal printing head. An apparatus according to claim 1, characterized in that said determining means comprises means for determining a printing density of a pixel on the basis of a distribution of the k image data items extracted by said extracting means.

Inventors: INOUE NOBUHIRO, INOUE, NOBUHIRO, INOUE, NOBUHIRO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1994
Main classification: G06K15
IPC codes: H04N1, G06K15, B41J2